A measurement of electromagnetic (EM) properties of earth formation penetrated by a borehole has been used for decades in hydrocarbon exploration and production operations. The resistivity of hydrocarbon is greater than saline water. A measure of formation resistivity can, therefore, be used to delineate hydrocarbon bearing formations from saline water bearing formations. Electromagnetic borehole measurements are also used to determine a wide range of geophysical parameters of interest including the location of bed boundaries, the dip of formations intersecting by the borehole, and anisotropy of material intersected by the borehole. Electromagnetic measurements are also used to “steer” the drilling of the borehole.
Borehole instruments, or borehole “tools”, used to obtain EM measurements typically comprise one or more antennas or transmitting coils which are energized by an alternating electrical current. Resulting EM energy interacts with the surrounding formation and borehole environs by propagation or by induction of currents within the borehole environs. One or more receivers respond to this EM energy or current. A single coil or antenna can serve as both a transmitter and a receiver. Parameters of interest, such as those listed above, are determined from the response of the one or more receivers. Response of one or more receivers within the borehole apparatus may be telemetered to the surface of the earth via conveyance means that include a wireline or a drill string equipped with a borehole telemetry system. Alternately, the response of one or more receivers can be stored within the borehole tool for subsequent retrieval at the surface of the earth.
Standard induction and wave propagation EM tools are configured with transmitter and receiver coils with their magnetic moments aligned with the tool axis. More recently, induction tools with three axis coils and wave propagation MWD or LWD tools with antennas (coils) whose magnetic moments are not aligned with the tool axis are being produced and used. These MWD or LWD propagation tools, with antenna dipole axes tilted with respect to the tool axis, can distinguish resistivity differences as a function of tool azimuth. Tools with coils aligned with the tool axis cannot distinguish resistivity changes as a function of tool azimuthal angle. The azimuthal resistivity response feature of an electromagnetic MWD or LWD tool is most useful in direction or “geosteering” the drilling direction of a well in a formation of interest. More specifically, the distance and direction from the tool to a bed (such as shale) bounding the formation of interest, or water interfaces within the formation of interest, can be determined from the azimuthal resistivity response of the tool. Using this information, the drill bit can be directed or “steered”, in real time, so as to avoid penetrating non hydrocarbon bearing formations with the borehole.
Prior art MWD or LWD tools that make azimuthal EM measurements employ a combination of separate axially aligned antennas and antennas whose magnetic moments are tilted at an angle with respect to the tool axis. Such tools, for example, are described in U.S. Pat. No. 6,476,609 issued to Bittar, and U.S. Pat. No. 6,297,639 issued to Clark et al. These tools have a fixed response azimuth, and can only preferentially determine resistivity or distance to a bed on one side of the tool, or at fixed azimuth angles relative to the tool. The tools must be rotated in order to respond to or “see” resistivity differences or boundaries on all sides of the borehole. Furthermore, the antennas with different dipole orientations located at different axial spacings along the length of the tool lack a common dipole origin point. This fact precludes vector addition of the dipole moments to form a new dipole moment, in any direction, with the same origin point. Multiple antennas at differing axial spacings also increase tool production and maintenance cost, and further reduces mechanical tool strength.
U.S. Pat. No. 6,181,138 issued to Hagiwara describes an antenna design that has three independent, co-located, antenna coils with a common tilt angle. Co-located in this context means having dipole moments whose origins are coincident and tilt angle is the angle between the tool axis and the dipole moment of the antenna coil. These antenna coils acting together can direct a transmitter or receiver resultant antenna dipole moment in any direction. This antenna design places coils around a drilling collar in a region of reduced diameter or “necked down” region. It is well known in the art that reducing the outer diameter of a drilling collar weakens it in that area and causes the collar to be more prone to mechanical failure. In this design also the coils must be covered with a non-conducting layer which must go all the way around the collar for the extent of the tilted coils. Non-conductive coverings presently used in the art such as fiberglass, rubber, epoxy, ceramics or plastic are subject to wear due to abrasion which occurs between the tool and the borehole wall, and are not as strong as the collar material. Because the non-conducting region must encircle the collar it is likely to contact the borehole wall unless the collar is further “necked down” causing further weakness. An extreme penalty is paid by “necking down” drilling tubulars. It is well known to those skilled in the art that reducing the outer diameter of a cylindrical member reduces the torsional and bending stiffness proportional to the forth power of the radius. For example, reducing the diameter of a 5 inch (12.7 centimeter) tubular to 4 inches (10.2 centimeters) reduces the torsional and bending stiffness by 59%.
U.S. Pat. No. 7,038,457 issued to Chen and Barber, and U.S. Pat. No. 3,808,520 issued to Runge, describe co-located triaxial antenna construction in which three orthogonal coils are wound around a common point on a borehole logging tool. They describe the virtues of having antennas with three orthogonal dipole moments all passing through the same point in the center of the logging tool. These patents are incorporated herein by reference. The teachings of both patents are more suitable for wireline tools because the disclosed coil windings would compromise the strength and durability of an MWD or LWD tool. Runge describes a triaxial antenna located in the center of a tool with non-conducting tool housing or “mandrel” around it. This is clearly not appropriate for MWD or LWD embodiment. It is known those of ordinary skill in the MWD or LWD art that a non-conducting tool body does not have the strength to support the severe mechanical requirements of tools used in drilling. Chen and Barber describe a technique for implementing an antenna structure with co-located magnetic dipole moments in which the transverse coils penetrate a mandrel through openings in the tool body. While this may be appropriate for wireline applications, openings in the tool body in which a coil is placed will cause weakness in the tool body. In addition provision must be made for drilling fluid or drilling “mud” to flow down within the body of an MWD or LWD tool. This mud usually flows in a conduit or channel in the center of the MWD or LWD tool, which is typically a drill collar. Embodied in a MWD or LWD system, the Chen and Barber design must somehow be modified to divert the mud away from the coils and the openings in the tool body thereby adding complexity and cost to the manufacture of the tool. Another problem encountered in embodying the Chen and Barber design as an MWD or LWD system is that, owing to the required non-conductive covering which is disposed around the circumference of the tool, the coils are not protected from abrasion which occurs between the tool and the borehole wall during drilling.
A crossed dipole antenna for wireline applications is described in U.S. Pat. No. 5,406,206 issued to Safinya et al. This patent describes a slot or cavity antenna with crossed magnetic dipoles and a dielectric backing. This system is purported to operate in the frequency range of 200 to 2000 MHz and as such would be only suitable as a pad mounted device which contacts the borehole wall during operation. The antenna described in the present patent can operate at frequencies below 10 MHz and does not require contact with the borehole wall for operation. The antenna of the present patent uses a thin plate of ferrite underneath current carrying wires and is much more efficient at lower frequencies
A more robust antenna design suitable for MWD or LWD application is described in U.S. Pat. No. 5,530,358 issued to Wisler et al. This antenna is integrated into a drilling tubular affording maximum strength and abrasion resistance, but it is only a single axial dipole antenna. One of the key components of the Wisler et al system is the antenna wire pathways disposed beneath the surface of the drilling tubular surface to avoid any abrasion and so as not to reduce the strength of the tubular. Protection of the coils and their non-conducting covering in the Chen and Barber design may be partially accomplished by reducing the radius or “necking down” the drilling tubular in the antenna region similar to the Hagiwara design. However “necking down” in the region of the antennas will significantly reduce the tubular strength in that region, and the tool will therefore be more prone to failure downhole. Although more robust, the Wisler patent does not teach varying the direction of the antenna dipole moment.